The RBL-2H3 and other cultured cell lines are used to study various aspects of endocytic and secretory processes. Emphasis is on the use of morphological, cytochemical, biochemical characterization of cultured cells. Events involved in receptor activation and signal transduction are being investigated as well as endocytic and secretory pathways. Studies also investigate the cell biology of these cells, the pathways for their maturation, and development. During the last year antibodies have been identified that bind to mast cell precursors and these have been used to purify and identify these cells.